Résumé : In situ hybridization of complementary [H3]DNA ([H3]cDNA) synthetized from purified rat prolactin messenger RNA (rPRL mRNA) was performed to specifically identify on histologic sections of rat hypophysis cells expressing the PRL gene. Radioautographic labelling occurred over weakly acidophilic cells, while other acidophils, with darker cytoplasm did not contain more silver grains than blood vessels.